1. Field of the Invention
This invention relates to a semiconductor device and a semiconductor device manufacturing method, and more particularly to, for example, a SRAM (Static Access Memory) and a semiconductor device with an SOI (Silicon On Insulator) structure where transistors are formed in a semiconductor layer on an insulating film.
2. Description of the Related Art
The impurity concentration in the channel region of an MIS (Metal Insulator Semiconductor) transistor formed on a semiconductor substrate is controlled, thereby adjusting the threshold voltage of the transistor. In recent years, semiconductor devices have been miniaturized further, leading to a decrease in the amount of impurities implanted into the channel region. As s result, it is difficult to control the impurity concentration, which makes variations in the desired impurity concentration significant. This gives rise to a threshold-voltage variation problem. A decrease in the amount of impurities leads to an increase in soft errors caused mostly by cosmic rays, which is a serious problem.
It is known that a fully-depleted SOI structure is effective in overcoming the difficulty of controlling the threshold voltage and decreasing the occurrence of soft errors. The SOI structure is a structure where transistors are formed in a semiconductor layer (e.g., silicon) provided on an insulating film. SOI-structure transistors are broadly divided into the fully-depleted type and the partially-depleted type. In the fully-depleted type, when a transistor is on, the semiconductor layer (or body region) under the channel is depleted completely. In the partially-depleted type, when a transistor is on, only a part of the body region is depleted.
With the fully-depleted SOI structure (hereinafter, just referred to as the fully-depleted type), the threshold voltage of the transistor is determined mainly by the work function of the gate electrode. This reduces variations in the threshold voltage.
There has been a hybrid semiconductor device where a memory transistor and a peripheral transistor constituting a peripheral circuit are formed on a semiconductor substrate (including a semiconductor layer of an SOI structure). Variations in the threshold voltage and the occurrence of soft errors can be considered to decrease by making the hybrid semiconductor device the fully depleted type.
As described above, in the fully-depleted type, the threshold voltage of a transistor is determined by the work function of the gate electrode. The work function is determined mainly by the material of the gate electrode. Thus, when a plurality of transistors have gate electrode made of the same material, these transistors have the same threshold voltage.
When memory cells of a hybrid semiconductor device are SRAMs, it is not required so much to set a plurality of threshold voltages to the threshold voltages of a plurality of memory cell transistors on a semiconductor substrate. Thus, even when the same material is used for the gate electrode of each memory cell transistor, no problem arises.
On the other hand, since the functions of the transistors constituting the peripheral circuit are diverse in the peripheral circuit, a plurality of peripheral transistors are required to have different threshold voltages. However, as described above, when the same material is used for the gate electrode of each peripheral transistor, a plurality of threshold values cannot be set. To solve this problem, the following approach can be considered: a plurality of materials with different work functions are prepared and the desired threshold voltage is set by selecting these materials suitably. This method, however, increases the manufacturing cost very much. Therefore, use of a plurality of gate electrode materials is not a practical solution to the problem.